Investigating the resistance of wires

Class practical

A simple investigation of the factors affecting the resistance of a wire.

Apparatus and materials

For each student group

Cells, 1.5 V, with holders, 2

Crocodile clips, 2

Ammeter (0 - 1 amp), DC

Leads, 4 mm, 5

Wire available for class use (see technical notes)

Power supply, 0 to 12 V, DC (OPTIONAL)

Metre rule (OPTIONAL)

Insulating tape (OPTIONAL)

Digital and analogue ammeters, 0-1 A (OPTIONAL)

Digital and analogue voltmeters, 0-12 V (OPTIONAL)

Micrometer (OPTIONAL)

Health & Safety and Technical notes

Modern dry cell construction uses a steel can connected to the positive (raised) contact. The negative connection is the centre of the base with an annular ring of insulator between it and the can. Some cell holders have clips which can bridge the insulator causing a 'short circuit'. This discharges the cell rapidly and can make it explode. The risk is reduced by using 'low power', zinc chloride cells not 'high power', alkaline manganese ones.

When using a power supply, high currents will cause the safety cut-out on the power packs to automatically switch it off. If short lengths of wire are used with relatively high currents and voltages, then significant electrical heating may also occur. Students should be encouraged to adjust the voltage to keep currents small with every set of readings. At each stage they can connect the circuit, take readings quickly and then disconnect the power supply.

If you use a mains power supply, use one that is designed to limit the output current to about 1 amp, and preferably with a current overload indicator.

Selection of reels of different wires (e.g. copper, Eureka, iron) of same gauge (e.g. 34 SWG).

Procedure

a Connect up a series circuit of two cells, and the ammeter, with a 30 cm length of one of the wires closing a gap between two crocodile clips. Note the reading on the ammeter.

b Replace the specimen of wire with another of the same length but different gauge or material.

c Investigate how the current depends on the thickness of the wire, its length and the material from which it is made.

Teaching notes

1 Use fine gauge wires. If too thick a wire is used, the results may be affected by warming of the wires.

2 If coils of copper and Eureka wires of the same gauge can be prepared so that they have equal resistances, the effect is very striking. However, this would then lose its value as an open investigation.

3 Students should come to understand that the resistance of a wire depends on its length, its cross sectional area, and the material out of which it is made. With some students you could go further and introduce the concept of resistivity ρ, through the relationship R = ρ l / A where R = resistance, ρ = resistivity, l = length and A = cross-sectional area.

4 This may also be an opportunity for a large scale demonstration of the effect by the teacher. But note: if the current is too large, the voltage of the cells will fall due to their internal resistance. For this reason, it is important to keep the current very low - copper wire is effectively a short.

5How Science Works extension: This experiment can be used as a more open-ended investigation. Students can select the variables, the ranges of results and the equipment used. The amount of guidance will depend greatly upon the teaching group. Investigating the effect of length on resistance is common but some students may wish to investigate the effect of the thickness of wire. In either case, different wires should be made of the same material. Students may need to know the conversion between SWG (standard wire gauge) and wire diameter/radius.

Students will find it easier to measure at a prescribed length if they tape the wire to a metre rule with insulating tape and make connections with flying leads rather than crocodile clips.